This invention relates to a process of preparing anhydrous alumina from aluminum hydroxide in a circulating fluidized bed comprising a fluidized-bed reactor 8, a separator 6, and a return line, where the aluminum hydroxide is introduced into a first suspension preheater 2 operated by means of the exhaust gases of the fluidized-bed reactor 8 of the circulating fluidized bed, and is at least partially dehydrated, dehydrated aluminum hydroxide from the first suspension preheater 2 is introduced into a second suspension pre-heater 5 operated by means of the exhaust gases of the fluidized-bed reactor 8 of the circulating fluidized bed, and is furthermore dehydrated and then supplied to the circulating fluidized bed, which is operated by means of oxygen-containing fluidizing gas 10 indirectly heated in a subsequent cooling stage by the alumina produced, and by means of directly heated, oxygen-containing secondary gas 11 supplied at a higher level, where the indirect heating of the fluidizing gas is effected in a fluidized-bed cooler 23. Such a process is described in DE-A-1592140.
As against the conventional processes using a rotary kiln and the processes carried out in what is called the classical fluidized bed, the above-stated process is in particular characterized by favorable heat consumption values of about 720 to 800 kcal/kg, which depending on the quality of the alumina produced are considerably lower than the typical values for rotary kilns in the range from 1000 to 1100 kcal/kg. These values are on the one hand reached as a result of a near-stoichiometric combustion of the fuel and the substantial utilization of the waste heat of the exhaust gases, which leave the calcining zone, for predrying and partial dehydration. On the other hand, the recirculation of the heat of the calcined material to the calcining zone in the form of the fluidizing gas and the secondary gas heated in the fluidized-bed cooler represents a substantial contribution to the reduction of the heat consumption values. A further advantage of the process consists in that due to the stepwise combustion, namely at first only by means of fluidizing air understoichiometrically in the range of a high dispersion density, and then in the presence of secondary air stoichiometrically or slightly overstoichiometrically in the range of a low suspension density, an overheating which would impair the quality of the product made by such process is definitely avoided.
What is disadvantageous in the above-described process is the fact that at the generally required high calcining temperatures of 1000 to 1100.degree. C. it is difficult to utilize the product heat in the actual calcining process. Either the gas streams required for a sufficient cooling of the product are so large that they cannot be completely used in the calcining process, or--in the case of a cooling against the gas streams required for the calcining process--the cooling of the product is not sufficient. Finally, the demands concerning the quality of the final calcined alumina have changed recently. What is desired in particular is an alumina of sandy quality, i.e. a high content of gamma-oxide. The changed requirements necessitate a considerable change in the execution of such process.